1. Prog.EneroyCombust.Sci. 1982, Vol. 8, pp. 317-354. 0360-1285/82/040317 38519.00/0
Printed in Great Britain. All rights reserved. Copyright © Pergamon Press Ltd.
URBAN A N D WILDLAND FIRE P H E N O M E N O L O G Y
F. A. WILLIAMS
Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, U.S.A.
to fire science. References 1 through 5 are examples of
1. I N T R O D U C T I O N
the type of material that is available. These references
Mankind's concern with unwanted fires likely pre-
are directed toward urban fires; fewer books are avail-
dates the first practical use of combustion in unre-
able concerning wildland fires. Reference 6 is a collec-
corded history. Yet the science of fire protection has
tion of articles concerning both urban and wildland
progressed more slowly than other aspects of com-
fires. Reference 7 often is cited as a forest-fire text.
bustion science. This state of affairs is due partially to
Reference 8 is a detailed documentation of many
the complexity of the problem and partially to the fact
forest fires that have occurred in North America.
that relatively large technological payoffs generally
Reference 9 is a fictional novel which nevertheless is
are not anticipated to be obtained from scientific
rather accurate technically, concerning a forest fire.
investigations of fires. Numerous promises and prob-
With the exception of the material in Ref. 6, the
lems in the use of combustion in heat and power pro-
level of scientific description in the books cited is not
duction, in locomotion and in industrial endeavors
very advanced. Typical undergraduates in engineering
have generated intensive scientific efforts. By way of
have backgrounds in physics, chemistry, thermo-
contrast, the ever-present fire problems have attracted
dynamics, fluid mechanics and heat and mass transfer
fluctuating interest with a relatively low average level
that would enable them to appreciate a more sophisti-
of concern.
cated treatment. No book exists giving a unified
Although periodic disasters engender beliefs that
exposition of fire science at a more advanced level.
more should be done in fire science, these beliefs often
The variety of authors of Ref. 6 provide material at an
are short-lived and are replaced by more immediate
advanced level with uneven coverage.
concerns. Rarely has a disaster or a series of disasters
A source of references on continuing research in fire
provoked a program of scientific study of fire phenom-
science is provided by Ref. 10. This periodic publi-
ena. The long-term efforts that have been mounted in
cation contains numerous good review articles on
the field instead have been motivated mainly by
various aspects of fire problems. Much of the informa-
detailed comparative evaluations of the magnitude of
tion underlying the present article has been obtained
the fire problem.
from Ref. 10. The biennial combustion symposia,
There is reason to believe that today's rapid techno- starting approximately with the tenth,11 contain many
logical advances intensify problems of unwanted fires.
original research papers in the field.
Hosts of new processes and new combustible materials
are emerging and finding their way into widespread
1.2. Magnitudes of Fire Problems
use. Too often these innovations become common-
place before their fire hazards are properly under- There have been many compilations of losses at-
stood. Therefore there seems to be justification for tributable to unwanted fires. The accuracy of such
information always is open to question because of
more extensive development and dissemination of
uncertainties in reporting and possible errors in collec-
knowledge in fire science.
tion and tabulation of data. Nevertheless, published
The present article has been prepared with the
numbers are at least roughly indicative of reality.
objective of bringing together many aspects of fire
science for a nonspecialized audience. It is based on an Table 1 lists some loss information obtained in the
undergraduate course by the same name, given only middle of the decade 1970. Some of the numbers given
once, at the University of California, San Diego. It have been rounded here in an effort to reflect uncer-
focuses on basic aspects of fire as a phenomenon, tainties.
Annual fire deaths per capita depend on many
presented in an elementary but unified manner. It is
factors, such as living conditions and degree of indus-
more restricted than other treatments of the subject in
trialization; they may vary over an order of magni-
that physiological, social, economic, organizational
tude. The relative significance of deaths and of prop-
and operational aspects are not covered. However, it
erty loss is difficult to assess rationally. In the United
is broader in that both urban and wildland fires are
States, most of the fires are residential fires, and most
considered equally; most presentations are slanted
of the fire deaths occur in residential fires, but most of
toward one or the other of these classes of fire
the property loss occurs in industrial and commercial
problems. I hope that this article will help to introduce
fires. In the figures for 1972, obtained by the Presi-
nonspecialists to the scientific phenomenology of fires.
dent's Commission on Fire Protection and Control,
1.1. Books on Fire Science the total cost is calculated as roughly equal contribu-
tions from property loss, fire-related expenses in build-
A number of books have been devoted specifically
317

2. F. A. WILLIAMS
318
TAnLE 1. Annual fire losses
US UK USSR Japan
Deaths 10,000 1,000
Deaths per million 40 12.6 1.2
Serious injuries 300,000
1972 property loss $3 X 10 9
Property loss capita $20 $2.5 $0.2 $2.6
1972 total cost $10 x 109
Number of fires 5 x 106
Fires fought by fire service per 1000
population 4.5 3.1 0.16
Forest acres burned (1973-78 average) 3.8 x 106
was record-breaking, but nevertheless about 80~o of
ing construction, costs of fire departments and fire
the population in the fire area survived in Hamburg.
insurance costs. The forest acres burned are approxi-
Large forest fires continue to occur when atmospheric
mately the size of the land area of the state of New
conditions favor them; the last entry in Table 2 repre-
Jersey. It appears from these figures that fire problems
sents 1260 separate fires in a two-month period, with
are significant, particularly in the United States.
total losses placed in excess of $2 x 1 0 6.
1.3. Historic Fires
1.4. Definitions of Fires
There are many well-known fires in recorded his-
Certain terms peculiar to fire studies deserve defini-
tory. A few of these are listed in Table 2, along with
tion at the outset. As a general definition, a fire may
some other fires that are not so well known. It is seen
be taken to be a chemical reaction of fuel with oxygen
that the famous fire of London burned over a relatively
to produce heat, thereby involving heat transfer and
small area. Fires often are associated with military
fluid flow. This definition is intended to exclude very
events; the Moscow fire coincided with Napoleon's
slow oxidations, such as rusting, but to allow for
occupation of the city. It is not generally known that
gaseous, liquid or solid fuels, polymers or metals,
in October, 1871, on the same day as the famous
burning under controlled or uncontrolled conditions.
Chicago fire, fires began relatively nearby, in the
A mass fire may be defined as any large fire involv-
Peshtigo area of Wisconsin and in central Michigan,
ing more than one sizeable structure and taxing
that burned through 17 towns and 5000 square miles,
resources of fire-fighting agencies. Mass fires may be
killing nearly four times the number of people who
divided into subcategories, depending on their charac-
perished in the Chicago fire; the coincidence likely
teristics. For example, a conflagration is a large propa-
reflects the occurrence of optimal weather conditions
gating fire; spread of the fire is a key aspect of this
for burning in the area.
definition. Large forest fires are typical examples of
Although the San Francisco fire, associated with an
conflagrations, but conflagrations also may occur in
earthquake, is well-known near the turn of the century,
cities. A fire storm may be defined as a large, intense,
the Baltimore fire was considerably more instructive
in revealing fallacious fire-fighting practice, x2 The localized fire, usually with a single convection column
above it, nonspreading and having high-velocity fire-
fires in Hamburg, Tokyo and Dresden, during World
War II, were caused intentionally by incendiary bomb- induced winds. Specific definitions of fire storms often
require the velocity somewhere to exceed a specified
ing; fire storms were established, and the loss of life
TABLE2. Some historic fires
Location Date Acres burned Homes lost Deaths
London 1666 336 13,200
Moscow 1812
Chicago 10/8/1871 2,124 300
Peshtigo 10/8/1871 3,600,000 1,000
Baltimore 1904
San Francisco 1906
Idaho and Montana 8/1910 3,000,000
Tokyo 1923 1,200
Hamburg 1943 2,500 40,000
Tokyo 1945 9,600 85,000
Dresden 1945 150,000
Hiroshima 1945 3,000
Ft. Yukon, Alaska 1950 2,000,000
Laguna, California 10/1970 175,000 382 3
California 9/15 11/15/70 600,000 885 14

3. Urban and wildland fire phenomenology 319
Table 3. A few combustible materials
Heat of combustion
Material Formula Flame color kcal/mole of fuel cal/g of fuel
Gases
Hydrogen H2 invisible 68.3 34,150
Carbon monoxide CO blue 67.6 2,410
Natural gas (methane) CH 4 blue 210.8 13,180
Propane C3 H8 blue yellow 526.3 11,960
Ethylene C2 H4 blue-yellow 337.3 12,050
liquids
blue-yellow 1149.9 11,500
Heptane C7H16
blue yellow 1302.7 11,430
Octane Cs H t s
Benzene yellow green 782.3 10,030
C6H 6
Gasoline HC 11,530
Kerosene HC 11,000
Methyl alcohol CH3 OH blue 170.9 5,340
yellow 1047.1 10,070
Styrene Cs Hs
Solids
Carbon (graphite) C yellow 93.9 7,830
C12H22011 1349.6 4,000
Sugar (sucrose)
397.2 4,460
Urethane C3H7NO 2
4,200
Cellulose (~ glucosan) C 6H 10O 5
Wood (birch, oak, etc. under
average conditions in nature) 4,000
Charcoal CH~(c~ < 1) 7,260
Steel (iron) Fe 1,580
Magnesium Mg 6,080
2.1. Hazard Aspects
value, e.g. 75 mph. Fire storms with well-developed
convection columns may generate clouds of water Many different properties of fuels have bearing on
droplets from condensation of cooled reaction pro- their fire hazards. One is their ease of ignition; even if
ducts; in extreme cases rain may fall from the clouds. its heat release is low, a material that can be ignited
A fire whirl or a fire vortex may be defined as a easily may pose a severe fire hazard. Another relevant
fluid-mechanical vortex with fire in it, generated or property is the heat of combustion, listed in Table 3;
intensified at least partially as a consequence of the materials with high heats of combustion can be rela-
fire. Fire whirls typically may be elements of fire tively more effective in sustaining fires. Flame spread
storms or of mass fires in general; they have been sug- is a third aspect of fire hazards; materials that are
gested as small-scale models for some types of fire difficult to ignite and that have low heats of combus-
storms. Intense whirls, sometimes called fire tornadoes, tion may nevertheless spread flames relatively rapidly
can be destructive. and thereby be dangerous.
Subsidiary aspects of behaviors of materials in fires
1.5. The Fire Triangle
also influence their fire hazards. Smoke can cause
Books on fire science often employ a triangle to damage and also can interfere with escape from fires
represent the key elements of a fire. The triangle has and with fire fighting; propensity of a material for
three legs, representing heat, air and fuel. Strategies smoke production therefore is relevant in assessing its
for fire suppression through flame extinguishment fire hazard. Materials capable of generating toxic pro-
often are viewed as attempts to remove one of these ducts in fires are of particular concern.
three elements. The fire triangle is intended to provide Finally, ease of extinction is a significant aspect
an intuitive feeling for essentials of fire at an elemen- of a material's fire hazard. An otherwise dangerous
tary level. material may be acceptable if its flames can be extin-
guished readily. There are many tricky aspects to the
2. COMBUSTIBLE MATERIALS
evaluation of fire hazards. Some will be considered
Of basic concern in fire problems is the identifica- later in connection with estimates of flammability.
tion of materials that can serve as fuel. Most things,
2.2. Fire Categories
even steel, will burn under suitable conditions; carbon
There is a partial correspondence of the states listed
dioxide, water and sand are examples of materials that
in Table 3 with the categories of fires employed in fire
cannot burn. Table 3 lists some c o m m o n combustible
protection. Fire classes are: Class A, Solid; Class B,
materials and gives some of their combustion proper-
Liquid; Class C, Electrical. These classes are defined
ties, notably the energy released when they burn.

4. 320 F.A. WILLIAMS
Pyrolysis means a chemical transformation pro-
roughly in increasing order of the danger associated
duced by application of heat. In finite-rate pyrolysis,
with the fire and call for different techniques of fire
molecules from the gas that strike the surface of the
fighting. There is no class corresponding to gaseous
fuel enter the condensed phase to a negligible extent,
fuels because fires with such fuels are encountered
and approximate formulas may be written directly for
relatively infrequently and because when they do
occur the duration of the fire usually is too short for the rate of gasification as a function of T. 15 A useful
counter-measures to be taken. This does not mean approximate formula for the gasification rate m, the
that gaseous fuels are less dangerous; on the contrary, mass per unit area per second of vapors leaving the
rapid flame spread through gases often generates surface of the fuel, is
pressure increases characteristic of explosions or buoy-
m -- msex p [ - E s / ( R T ) ] , (2)
antly rising clouds of burning gases with damaging
levels of radiant energy transfer. where ms and Es are constants, the latter being an
In all three fire classes usually gases actually burn. effective overall energy of activation for surface pyro-
These gases are secondary, not primary fuels and are lysis. Values of ms and E~ are difficult to find in the
liberated from the liquid or solid fuels in the fire literature, although some information is available.6
environment. There are a few exceptional cases in All of the gasification parameters that have been
which the liquid or solid fuels burn directly without introduced here are properties of fuels. Understanding
previous liberation of combustible gases. Carbon, of these properties and of pyrolysis processes requires
some explosives and solid propellants, and certain knowledge of chemical bonding, chemical conversion
metals are examples of fuels that burn directly, and and heat liberation.
glowing combustion of wood or tobacco is a burning
process that does not involve a gaseous combustible 3. CHEMICAL CONVERSION A N D HEAT LIBERATION
intermediary. Chemical conversion is a process whereby a chemi-
cal in one form is transformed into chemicals in other
2.3. Burning Mechanisms of Solid and Liquid Fuels
forms. The many different types of chemical con-
It is important to understand the usual mechanism, version that are possible are dictated by molecular
alluded to above, by which condensed-phase fuels structure, which is determined by chemical bonding.
(solids and liquids) burn. The heat is released in the
3.1. ChemicaI Bonding
gas phase by the exothermic combustion of the secon-
dary gaseous fuels. Some of this heat is transferred Molecules are formed by establishment of chemical
back to the condensed phase to cause gasification of bonds among atoms. These bonds may be ionic (i.e.
the primary fuel. This gasification usually is an endo- involve exchange of electrons) or covalent (i.e. involve
thermic process (requiring heat) which releases the sharing of electrons). For combustible materials co-
gaseous combustibles to burn. Thus, feedback of heat valent bonds are by far of greatest importance. Ar-
from the gas-phase flames to the condensed-phase rangements of bonds formed in stable molecules de-
fuels usually is an essential aspect in maintaining a pend on the valences of the atoms involved. A covalent
fire. bond, two shared electrons, conventionally is indi-
cated by a bar; for example, H 2 is H--H. Some of the
2.4. Gasification Processes
fuels in Table 3 are
Two fundamentally different types of gasification
H H H
processes occur in fire. One, encountered most often
I I I
for liquid fuels, is equilibrium evaporation, and the
methane, H--C--H ethylene, H--C=C--H
other, encountered most often for solid fuels, is finite- I I [
rate pyrolysis. H H H
In equilibrium evaporation, interphase equilibrium
is maintained at the surface of the fuel. This equi- H
I
librium may be described by a useful approximate
methanol, H--C--O--H
formula for the mole fraction X~ of fuel vapor in the
I
gas at the surface of the condensed fuel. 13 If T(K) is H
the surface temperature, Tb(K) the normal boiling
temperature, R ~ 2 cal/mol K the universal gas con- H H
I I
stant and L the latent heat of vaporization, then
CmC
// %
L 1 1
and benzene, H - - C C--H,
/
C=C
Tables of L and Tb are available; 13'14 for example
I
L ~ 10kcal/mole and Tb = 373K for water. Equa-
H H
tion (1) thus provides a relationship between Xe and
T. This relationship must be used in formulas for which will be represented as H - - © for brevity. Note
transfer rates to obtain gasification rates under con- that ethylene, for example, has a C = C double bond,
ditions of equilibrium evaporation. representing four shared electrons.

5. Urban and wildland fire phenomenology 321
3.2. Polymers H
I
Many of the solid fuels of concern in fires are H--C--O--H
i
polymers. Polymers are large molecules, formed con-
C O
ceptually (in the simplest cases) by breaking a double
bond in identical molecules and interconnecting
them. 16 Thus, polyethylene is C C
-o / ?-H
H H H HIH H
C C
J lli i
I I
.... C m C - ~-C--C ,--~-C--C • •.,
H O--H
i II I:1 I
H H',H H
H H
Qualitative feelings for burning behaviors of
cellulosic materials are common. There is less in-
where the broken vertical lines separate the ethylene tuition concerning burning behaviors of synthetic
quot;monomersquot;. The degree of polymerization is the polymers. Many, such as polyethylene, polystyrene
number of monomers in the polymer chain; the chains
and poly(methyl methacrylate) soften and form a
may be terminated in various ways, e.g. by placing an liquid-like quot;meltquot; when they burn. Thus, it becomes
H at the end. unclear as to whether they should fall in fire class
Styrene is
A or B. Polyvinylchloride may form corrosive HCI
during combustion, while acrylonitrile may produce
H H
measureable amounts of highly toxic HCN upon
pyrolysis. Thus, the advent of synthetic polymers
C=C ,
f J raises new fire problems.
© H
3.3. Bond Energies
and therefore polystyrene is Energetic aspects are of importance for chemical
conversions that occur in fires. Energies liberated in
H H H H chemical processes, such as heats of combustion, need
I I I I to be known. Energies absorbed, such as heats of
.... C--C--C--C ....
I I i i pyrolysis of polymers, energies required to convert
0 H 0 H specified polymers to gases at a given temperature,
also must be known. There are many tables 14 of these
In addition to polyethylene and polystyrene, many heats of reaction. However, often it is of interest
other synthetic polymers are experiencing increasingly to calculate energy changes for processes that are
widespread use. These include polyvinylchloride difficult to find in tables. Bond-energy methods enable
such calculations to be performed, with accuracies
that although typically are not high nevertheless are
sufficient for many purposes.
H H
The bond-energy approach rests on the idea that a
I I
(monomer unit - - c - - c - - ), definite amount of energy liberation is associated with
I I the formation of a given chemical bond. The idea is
H C1 not precisely correct in that energy liberated may
depend also on the molecule in which the bond occurs
acrylonitrile and on the location of the bond within the molecule.
In fact there are correction procedures to account for
H H these effects, which may be quite substantial. In a
I I rough first approximation the corrections may be
--c--c-- )
(monomer unit
neglected, and the energy liberated in forming a
I I
H C~N gaseous molecule from its constituent atoms may be
calculated simply by adding the energies associated
and poly(methyl methacrylate), quot;plexiglasquot; or quot;lucitequot;, with each bond formed. A list of the bond energies
with monomer unit needed for this calculation is given in Table 4, which
has been taken from information in Ref. 17 and does
I not necessarily represent the most up-to-date infor-
H H--C--H H mation, although it is useful for illustrative examples.
I I I Accuracies in energy calculations better than 5 0 ~
H--C C C--O--C--H.
may be anticipated when using Table 4.
i f fi i
H O H
3.4. Combustion Reactions
The combustion reaction which occurs in the flames
Cellulose, the principal polymeric constituent of
of fires is a chemical combination of fuel with air
natural wood, is built from a glucosan monomer,

6. 322 F.A. WILLIAMS
3.6. Flame Temperature
TABLE4. Mean bond energies (kcal/mole)
Temperatures of flames exceed ambient tempera-
Bond Energy Bond Energy
ture because the heat released in combustion goes into
C--C 85 N=-N 225 raising the temperature of the combustion products.
C~---C 143 H--H 103 The extent to which the temperature is raised depends
C~C 198 O--H 109
on the heat capacity Cp of the products. Tables of cp
C--H 98 O--N 150
are available.'4 In fact cp varies with temperature, but
C--O 86 N--H 88
as a first approximation it may be taken as constant.
C=O 173 S--S 50
C--N 81 C1--C1 57 For gases cp generally lies between 0.2 and 0.5 cal/gK;
C~N 210 Br--Br 46 in a very rough approximation it may be taken as
C--CI 78 I--I 36
0.3 cal/gK for all gases. For liquid water % - 1 cal/gK;
C--Br 67 F--F 36
for most other liquids and for solid combustibles it
C--I 64 H - - C1 103
C--F 102 H--Br 88 typically lies between 0.3 and 0,7 cal/gK.
C--S 64 H--I 72 F r o m the molar heat of combustion Q, the heat
O--O 33 H--F 135
release per unit mass of products may be calculated as
O=O 117 H--P 76
Q/W, where W is the sum of the molecular weights of
N--N 60 H--S 81
the species on the right-hand side of the equation for
the chemical conversion of one mole of fuel, i.e. the
stoichiometric mass of all products per mole of fuel
consumed. The flame temperature TI is then found
to produce CO2, H 2 0 , N2 and heat. Air, in a
from the adiabatic energy balance Q/W = cp(Ty- Ti),
first approximation, is 0 2 + 4N 2. Thus, for example,
where T~ is the initial temperature, typically r o o m
the combustion of hydrogen in air is represented
temperature, about 300K. Thus
as H z + ½ O z + 2 N 2 ~ H z O + 2 N a + Q H 2, where Qn~
is the heat of combustion per mole for hydrogen. (3)
Ts = T , + Q / t W c p ) .
Similarly, for carbon monoxide, C O + ½02 + 2N2
Corrections to this for phase changes may be included
CO2 + 2 N z + Q c o . These equations are balanced
by suitably revising Q.
chemically in that there is no fuel or oxygen left over;
As an example, consider the combustion of propane
such chemical conversions are termed stoichiometric.
in air, C3H s + x(O 2 + 4N 2) --~3CO 2 + 4 H 2 0 + 4xN 2 +
Balancing a chemical reaction to achieve stoi-
QC3H~ with x = 5 from the chemical balance. F r o m
chiometry may be illustrated by considering the
Table 3, since the molecular weight of propane is
combustion of heptane. Write the reaction as C7 H ~6 +
44 g/mole, QC3H, = 11,960 × 44 = 526,000 cal/mole,
x(O2 + 4N2) ~ 7CO2 + 8 H 2 0 + 4xN2 + QCTH~6,
and W = 3 × 4 4 + 4 × 1 8 + 2 0 × 28 = 764 g/mole. Hence,
where x is unknown. The coefficients of C O 2 and of
with ee = 0.3g/mole K, eq. (3) gives Ts = 3 0 0 + 6 9 0 /
H 2 0 have been determined from the chemical formula
0.3 = 2600K, which is about 300K too large. This pro-
of the fuel. An oxygen balance then is used to find that
cedure usually overestimates TI because it neglects
x = 11, thereby completing the stoichiometry.
effects of dissociation of reaction products, which
occurs above about 2000K; dissociation involves, for ex-
ample, C O 2 ~ - C O + ½0 2. There are iterative methods
3.5. Calculation of Heat of Combustion
and computer programs for calculating Ty with dis-
The energies Q in the preceding equations are
sociation included (see, for example, Ref. 18). A short
best calculated from tables of standard heats of for-
mation, the energies liberated when molecules are
formed from their constituent elements in their stan-
TABLE5. Approximate flame temperatures of various
dard states. A somewhat less involved approach is
stoichiometric mixtures having initial temperature 298K
to use the bond energies listed in Table 4. As a
simple example consider the combustion of hydro- Pressure
Fuel Oxidizer (atm) Tf(K)
gen. Write the equation for chemical conversion as
H--H+½0 = O~H--O--H+Qrc F r o m Table 4,
Acetylene Air 1 2600*
this implies 103+½× 117 = 2× 109-QH2, where
Acetylene Oxygen 1 3410quot;
additivity of energies in reactions has been employed. Carbon monoxide Air 1 2400
The negative sign occurs because the heat of combus- Carbon monoxide Oxygen 1 3220
Heptane Air 1 2290
tion is positive if the total bond energies of the
Heptane Oxygen 1 3100
products exceed those of the reactants. The result that
Hydrogen Air 1 2400
QH~ = 56.5kcal/mole for combustion of gaseous H 2 Hydrogen Oxygen 1 3080
with gaseous 0 2 to form gaseous H 2 0 is within 5 ~o of Methane Air 1 2210
Methane Air 20 2270
the correct value. To find QH2 for combustion to
Methane Oxygen 1 3030
liquid H 2 0 , the latent heat of vaporization L must be
Methane Oxygen 20 3460
added to this result. The accuracy obtained here is
better than average; it is preferable to use tables for Q *A maximum temperature that occurs under fuel-rich
if they are available. rather than stoichiometric conditions.

7. Urban and wildland fire phenomenology 323
table of adiabatic flame temperatures, taken from may break another, stable polymer somewhere in the
Ref. 18, is shown in Table 5. It is seen that tempera- middle, forming a new stable polymer with half of the
tures in the range of 2300K are typical for burning in attacked chain and leaving the other half active.
air. These temperatures are of importance in calculat- Among the possible termination steps is direct
ing heat transfer in fires. The method for calculating combination of the radicals at the ends of two active
Ts that has been outlined here is useful for obtaining chains to form a single stable polymer. Another type
quick rough estimates. of termination step is disproportionation, in which
two active radicals deactivate each other by an ex-
change at the end of the chain. For polystyrene, an
4. CHEMICAL KINETICS OF PYROLYSIS
example of disproportionation may be
Other chemical conversions, in addition to combus-
H H H H H H H H
tion reactions, can be of significance in fires. These I I [ I I I J I
include the formation of smoke and of toxic products ...--C--C--C--C-- +--C--C--C--C--... ,
I I I I I I I I
in flames as well as conversions of solid fuels to
OH OH 0 H 0 H
gaseous combustibles. Many of these reactions are
pyrolysis processes. For example, smoke may be pro- H H H H H H H
I I I I I I I
duced through a sequence of pyrolysis reactions of
...--C--C--C=C + H--C--C--C--C--....
gas-phase fuels in fuel-rich regions, and carbonaceous
I I I I I I [ I
residues may arise from liquid-phase pyrolysis of © H © H © H © H
heavier liquid fuels. Attention here is focused on pyro-
Clews concerning pyrolysis mechanisms for specific
lysis of solid fuels to produce gaseous reactants.
polymers are obtained from many different experi-
4.1. Chain Reactions in Polymer Pyrolysis mental observations. 15 One such measurement is the
percentage of product volatiles composed of monomer,
The chemistry of polymer pyrolysis is complex and
found when the polymer is heated in a vacuum. Some
differs for different polymers. Simplified descriptions
data of this type are given in Table 6, taken from
are needed to achieve understanding. A useful simplifi-
Ref. 15. If the monomer yield is low then unzipping
cation for many processes of polymer pyrolysis (as
is unlikely, while high monomer yields are consistent
well as for the kinetics of combustion reactions them-
with unzipping.
selves) is the idea of a chain reaction. Chain reactions
have active intermediate species, chain carriers, whose
4.2. Simplified Kinetic Expressions
presence cause the reaction to proceed more rapidly
than it otherwise would. The chain carriers are formed Rates of polymer pyrolysis may be described by
in initiation steps, cause the reaction to proceed in expressions for dM/dt, the time rate of change of the
chain-carrying or propagation steps, and are con- mass M of the condensed phase in a homogeneous
sumed in termination steps. system. Such expressions may be complicated for
For polymer pyrolysis, there are many types of ini- chain reactions. There are conditions under which
tiation steps. In end initiation, the monomer at the useful simplified approximations may be obtained.
end of the polymer chain splits off, leaving a radical (a For example, for an unzipping process with a kinetic
species with an unsatisfied chemical bond) at the chain length (or zip length, i.e. the number of propa-
chain end. In random-scission initiation, thermal fluc- gation steps that occur prior to termination) com-
tuations break the polymer at random points along its parable with the degree of polymerization, each initia-
chain, producing radicals on each side of the scission. tion effectively results in unzipping of an entire chain.
In weak-links initiation, the polymer is broken in- The rate of mass loss then is controlled by the rate of
ternally at preferred high-strain spots, again leaving initiation, and
radical-ended chains.
dM/dt = - k M , (4)
There are also many types of propagation steps. A
relatively easy type to understand is unzipping, in where k is a specific reaction-rate constant for initia-
tion. Often this first-order reaction-rate expression
which a single monomer unit is formed and detached
provides a reasonable approximation under more
at the radical end of the chain. An illustration of un-
zipping for polystyrene is complex circumstances, in which k becomes an effec-
tive rate constant that includes influences of many
H H H H H H H H
different steps.
I [ I I I I [ I
The rate constant k depends on temperature T.
, ...--C--C--+ C = C .
...--C--C--C--C--
I 1 I I 1 1 J I Often an Arrhenius expression for this dependence
0 H 0 H 0 H 0 H provides a good approximation. Thus,
Propagation steps other than unzipping could be k = Bexp [-Eb/(RT)], (5)
intramolecular transfer steps, namely detachment of
where B and E b are constants, the latter being the
higher units of the monomer, e.g. dimers or trimers,
overall activation energy for bulk degradation. A
from the radical end. Intermolecular transfers, inter-
table of some measured values of Eb is shown as
chain propagation processes, also are possible. For
Table 7, again taken from Ref. 15. There have been a
example, the radical at the end of an active chain

8. F. A. WILLIAMS
324
TABLE6. Yield of monomer in the pyrolysis of some organic polymers in a
vacuum
Temperature Yield of
range monomer, ~o
°C of volatiles
Polymer
Polymethylene 335-450 0.03
Polyethylene 393-444 0.03
Polypropylene 328-410 0.17
Polymethylacrylate 292 399 0.7
Hydrogenated polystyrene 335 390 1
Poly(propylene oxide), atactic 270-550 2.8
Poly(propylene oxide), isotactic 295-355 3.6
Poly(ethylene oxide) 324-363 3.9
Polyisobutylene 288-425 18.1
Polychlorotrifluoroethylene 347-415 25.8
Poly-fl-deuterostyrene 345 384 39.7
Polystyrene 366-375 40.6
Poly-m-methylstyrene 309 399 44.4
Poly-~-deuterostyrene 334-387 68.4
Poly-~,fl, fl-trifluorostyrene 333-382 72.0
Poly(methyl methacrylate) 246 354 91.4
Polytetrafluoroethylene 504-517 96.6
Poly-ct-methylstyrene 259 349 100
Polyoxymethylene Below 200 100
number of studies in which expressions for k have Two conceivable paths are
been derived for more complex mechanisms. ~9 It can
k~ nCO+nH 2 +nO z
often be shown for steady-state pyrolysis that the E~ in
eq. (2)of Section 2.4 is Eb/2. ....-/'/' ~ nCO2 + n H 2 0
(CH20)n k 2 quot; ~ n C+nH20 +.~nO2
4.3. Competition in Pyrolysis
Certain materials such as wood and paper exhibit The rate constant for the initial step is kl in the upper
two types of combustion, flaming and glowing. The path and k 2 in the lower. The final two arrows
occurrence of these two types may be traceable to the represent oxidation, involving combination with 0 2
existence of two competing pyrolysis mechanisms for to produce combustion products.
Although the final products of combustion are the
the fuel. Such competition may be illustrated most
simply by considering pyrolysis of a carbohydrate, the same, the different intermediaries can cause the burn-
formula for which is (CH20)n, with n = 6 for glucose. ing mechanisms to differ. The species C O and H 2 are
TABLE7. Activation energies of thermal degradation of some organic polymers in a vacuum
Temperature Activation
Molecular range, energy
Polymer weight °C kcal/mole
Phenolic resin -- 332-355 18
Atactic poly (propylene oxide) 16,000 265-285 20
Poly(methyl methacrylate) 150,000 226-256 30
Polymethylacrylate -- 271-286 34
Isotactic poly(propylene oxide) 215,000 285-300 45
Cellulose triacetate -- 283-306 45
Poly(ethylene oxide) 10,000 320 335 46
Polyisobutylene 1,500,000 306 326 49
Hydrogenated polystyrene 82,000 321-336 49
Cellulose -- 261 291 50
Polybenzyl 4,300 386-416 50
Polystyrene 230,000 318 348 55
Poly-~-methylstyrene 350,000 229 275 55
Poly-m-methylstyrene 450,000 319-338 56
Polyisoprene -- 291 306 57
Polychlorotrifluoroethylene 100,000 332 371 57
Polypropylene -- 336-366 58
Polyethylene 20,000 360-392 63
Poly-e-fl-fl-trifluorostyrene 300,000 333-382 64
Polymethylene High 345-396 72
Poly-p-xylyene -- 401-411 73

9. Urban and wildland fire phenomenology 325
bustible, and the char that remains can support only
a surface oxidation, glowing combustion. Estimates of
rate constants, according to eq. (5), are B2 = 10 ~2 s-
and Eb2 = 40 kcal/mole for k2, the char process, and
B 1 = 1 0 1 7 s - 1 and Ebl = 53kcal/mole for kl, the tar
process.*
A reasonable mechanism has been suggested for the
tar-production path. 2° The yield of levoglucosan is so
high that probably some sort of an unzipping process
is indicated. It has been proposed that the chain may
rc quot;r
be initiated either by random scission or by end-
initiation, through attacks by a hydroxyl group, OH,
FIG. 1. Illustration of competing rates of pyrolysis.
one of which is attached to the C atom at the end of
each chain. After the monomer breaks off, propagation
gaseous fuels and therefore may escape from the solid
could be sustained by the free oxygen bond. It is the
and support flaming combustion. By contrast, in the
reason for the monomer appearing as levoglucosan
lower (dehydration) path H 2 0 is noncombustible
which requires explanation. A proposed model for
while C is a solid. The lower path therefore does not
this process is a two-step attack, 2° viz.,
liberate gaseous combustibles but instead forms C
which experiences surface burning, a type of glowing
combustion process of the solid fuel. While tobacco H2COH
burns by a process analogous to the lower path, [
0
matches burn by processes corresponding to both /t C
paths, the flaming resulting from a process like the /H
/~Cellulose
upper path. H C
C
H/I H
With the two competing processes illustrated, the _O~/ O1-1
rate of conversion of the fuel is
C
(6) C
dM/dt = - (k 1 + k2)M,
J I
in which kl and k 2 are given by separate expression of H OH
the type shown in eq. (5). It may be seen that if the
activation energies differ, Eb~ ~ Eb2, then different
H2COH
reactions may predominate at different temperatures.
f
This is illustrated in Fig. 1. At sufficiently low T, both
rates are negligibly small. Typically k doubles when T
increases by an amount on the order of only 10°C. At
slightly elevated temperatures, k 2 may be appreciable
while kl is negligible. Above T~, k 2 soon becomes
H C H~-'~ 0 ~ C d Cellulose
yf
negligibly small compared with k 1. For cellulosics, k2
corresponds to dehydration and k I to production of
secondary fuels capable of burning in the gas phase.
C C
I I
4.4. Pyrolysis of Cellulosics
H OH
Pyrolysis mechanisms of cellulose have been sub-
jected to detailed investigation. Numerous techniques
have been employed, and a multitude of facts have H2 C -O
o
been established. Although the current situation is I
complex, a few unifying principles have been de- C
vel°ped'2°'21 In particular' there appear t° be tw° /H I
principal competing paths, which may be represented H
,/
as , C C ~ + Cellulose
H
H
quot;dehydro- HO / ~OH
t200- L-,~ll, lnse '' +HzO----~har + H 2 + C O 2 +...
(exothermic) C
54~¢~oc'~t. / ~'~i~,'~i, C
. . . . , ~2/ en~to.i~ermic) I I
cellulose- OH
(280- k ~ (endothermic) H
340°C) quot; t a r ' (primarily
--levoglucosan) * These values are approximations to those of A. Broido,
reported in quot;Kinetics of Solid-Phase Cellulose Pyrolysisquot;,
The quot;'tarquot; is volatile and vaporizes to form a major (see Thermal Uses and Properties of Carbohydrates and Lignins
gaseous fuel to support a gas-phase flame. The gases (F. Shafizadeh, K. V. Sarkanen and D. A. Tillman, eds.),
Academic Press, New York, 1976).
evolved in the dehydration path are mainly noncom-

10. 326 F.A. WILLIAMS
first by the oxygen radical and next by the hydroxyl.
The final molecule shown is levoglucosan (fl-glucosan
or 1,6 anhydroglucose). The first step is endothermic
and the second exothermic, releasing less heat than is
required for the first step.
For the dehydration process, it has been reasoned 2°
that an out-of-plane, interrnolecular interaction must
be the cause. The hydroxyl in an H2COH group of
one chain can attack the carbon-oxygen linkage of an
adjacent chain, breaking that chain in such a way that
half of it is linked to the attacking chain while the
other half gives up H 2 0 in forming a stable end-
group. Hypotheses for the mechanism of the further
decomposition toward char through production
of HzO and CO have also been developed. 2° Thus,
the dominant features of the pyrolysis of pure cellu-
lose can be understood self-consistently.
--- WICK
Although cellulose is the major constituent of cellu-
losics such as natural woods, there are other impor-
WAX
tant constituents, notably hemicellulose and, typically
in somewhat lower concentration, lignin. 22 These
materials have less regular structures than cellulose
and show more complex behavior upon pyrolysis.
Even cellulose has a macrostructure, exhibiting amor-
phous regions and more regular crystalline segments.
This macrostructure may affect pyrolysis behavior.
Small amounts of inorganic constituents also have FIG. 2. Schematic illustration of burning candle.
measurable influences on pyrolysis. Therefore the
overall kinetics of thermal degradation of natural reactant molecules. For example, for A + B ~ p r o -
cellulosics vary. Nevertheless, the pyrolysis properties ducts, the rate co (moles of A consumed/vol, s) is co =
of cellulose always exert an influence on the rates of kCACB, where the rate constant k may be given by an
breakdown of cellulosics subjected to heat, and cellu- expression like eq. (5). Table 8, taken largely from Ref.
lose provides the best model currently available for 18, lists approximate rate constants for a few elemen-
these natural substances with respect to their pyrolysis tary steps.
kinetics. The species CH 3 and H are radicals that serve as
chain carriers. The first two reactions in Table 8 are
representative initiation steps, with M denoting any
5. CHEMICALKINETICSOF COMBUSTION
stable molecule. In established flames these steps may
The mechanisms of gas-phase reactions occurring be relatively unimportant since radicals H, O and OH
in fires may be discussed by reference to the burning may reach the fuel molecules by diffusion and consume
of a candle, illustrated in Fig. 2. The hydrocarbon fuel them more rapidly by propagation steps such as 3, 4
(wax) vaporizes from the wick under the influence of and 5. It is known that formaldehyde, H2CO , plays a
the heat from the flame. The dark region is fuel rich role in hydrocarbon oxidation, and step 6 is a potential
with insufficient oxygen for appreciable oxidation. means for producing it. Steps 7 and 8 describe a path
The blue is characteristic of the burning zone where for production of CO through the formyl radical
gaseous fuel meets oxygen; the blue colour is chemi- (HCO). Oxidation of CO to CO 2 occurs by step 9,
luminescent, not thermal or equilibrium radiation but which may proceed more slowly than other steps,
rather nonequilibrium radiation from species that leaving unburnt CO if reactions are quenched by
have achieved excited states through the chemical rapid cooling. Steps 10 through 13 are part of the
reactions of combustion. The yellow is mostly equi- chain mechanism for hydrogen oxidation and are
librium radiation from fine, hot soot particles that quite relevant to hydrocarbon oxidation. The last
may be burning with oxygen; the soot has been reaction listed is a representative termination step,
formed by pyrolysis of fuel gases. Chemical processes involving three-body collisions and having a rate pro-
that occur in the blue flame have been subjected to portional to the product of the concentrations of the
detailed investigation. three reactants.
5.1. Mechanisms and Rates in Methane Flames 5.2. Simplified Rate Expressions
Combustion reactions fundamentally are chain re- Many steps not shown in Table 8 are known to
actions involving many elementary steps. Each step occur in methane oxidation. Gas-phase oxidations of
proceeds at a rate proportional to the product of other fuels involve many additional steps as well.
the concentrations c (moles/vol.) of the colliding Knowledge of rates of elementary steps and computer

11. Urban and wildland fire phenomenology 327
TABLE 8. A few rate constants for reaction steps
Reaction k-Rate constant*
1.5 x 1019exp ( - 100,600/RT)
1. CH4+M~CHa+H+M
1.0 x 1014exp ( - 45,400/RT)
2. CH4+O2--*CH3 + HO2
3. CH4+O~CH3 +OH 1.7 × l0 la exp ( - 8,760/RT)
6.3 x 10la exp ( - 12,700/RT)
4. CH4+H~CH3+H 2
2.8 x 1013exp ( - 5,000/RT)
5. CH4+OH~CHa +H20
1.3 x 1014exp ( - 2,000/RT)
6. CH3 + O ~ H 2 C O + H
2.3 x 1013exp(- 1,570/RT)
7. H2CO + O H ~ H C O + H 2 0
1.0 x 1014
8. HCO+OH~CO +H20
3.1 x 1011exp ( - 600/RT)
9. CO+OH~CO2+H
2.2 × 1014exp ( - 16,600/RT)
10. H+O2-,OH+O
4.0 × 1014exp ( - 9,460/RT)
It. O+H2~OH+H
8.4 × 10X'~exp( - 18,240/RT)
12. O+H20~2OH
1.0 x 10X4exp( - 20,400/RT)
13. H + H 2 0 ~ H 2 +OH
14. H+OH+M~H20+M 2.0x 10~ T -l**
* Units are cm3/mole s.
** Units are cm6/mole2s for k and K for T.
Complete chemical equilibrium would involve equi-
capacities are becoming sufficient to enable compu-
librium for every step, a condition seldom achieved.
tations of histories of chemical conversions to be
However, equilibrium often is a good approximation
made with full chemistry for most fuels. However, for
for certain steps involving major species such as H 2 0 ,
many purposes it is helpful to have simplified expres-
sions for overall rates of heat release involving a small CO2 and CO. Equating forward and backward rates
n u m b e r of lumped steps that are not elementary, e.g. results in a relationship between concentrations and
expressions corresponding to two overall steps, first temperature for equilibrium (see Ref. 18, for example)
that involves an equilibrium constant, K c = kl/kb,
combustion of fuel to CO and H 2 0 then oxidation of
CO to CO,. Overall rate parameters for such simpli- where k s and k b are the previously defined rate
constants for the forward and backward elementary
fied descriptions are becoming available (e.g. Ref. 23).
steps. Combining such equilibrium equations with
For many purposes, a one-step approximation to
equations for element conservation (stating that
the complex chemistry is sufficient. The molar rate of
chemical elements are neither created nor destroyed
consumption of fuel F by oxidizer O is represented, for
in chemical reactions) and for energy conservation
example, as
results in expressions for temperature and for concen-
dcF/dt = - w = - c F c o B e x p [ - E / ( R T ) ] , (7)
trations of major species as functions of a local
mixture ratio (total local concentration of an element
in which the overall activation energy E and the
contained in the fuel, divided by total local concen-
overall prefactor B are constants. Over a sufficiently
tration of the element oxygen) in diffusion flames.
limited range of conditions, a representation of the
These expressions often are obeyed, in a rough ap-
type shown in eq. (7) often is acceptable.
proximation, in fires.
5.3. Chemical Equilibrium
5.4. An Example of Diffusion-Flame Structure
There are situations in fires under which chemical
These ideas of chemical equilibrium help to ex-
rates for combustion need not be considered at all
plain some major observed characteristics of diffusion
because, in a first approximation, chemical equi-
flames. The shape of the blue flame in Fig. 2 causes it
librium is attained locally at each point in the gas.
to be difficult to probe. Measurements are easier to
These situations may occur only in nonpremixed
perform in flat diffusion flames, which may be estab-
systems (systems in which the fuel and air are not
lished with the apparatus illustrated in Fig. 3. 24 A
mixed prior to burning), often termed diffusion flames
liquid fuel is contained in a pool (shaded), and an
since burning then involves diffusion of fuel and
oxidizing gas stream is directed downward onto the
oxidizer toward each other. They cannot occur every-
surface of the liquid. When the fuel is ignited, con-
where in premixed systems (systems in which fuel and
ditions can be adjusted so that a flat flame remains
oxidizer are mixed at a molecular level) because the
stationary a few millimeters above the surface of the
equilibrium state involves negligible concentrations of
fuel, as illustrated. Quantities vary only in the vertical
either fuel or oxidizer. The system illustrated in Fig. 2
direction, and the flame structure may be studied by
is nonpremixed and therefore subject to approxima-
thermocouples and by gas sampling. The liquid fuel
tion by chemical equilibrium; in fact, most fires involve
may be replaced by a gaseous fuel jet or by a solid fuel.
diffusion flames.
Representative results for the flame structure in
At chemical equilibrium for a reaction step, the
such an apparatus are shown in Fig. 4, for the solid
forward rate equals the rate of the backward reaction
fuel poly(methyl methacrylate). The gas stream had
(defined by reversing the arrow, e.g. in Table 8).

12. 328 F.A. WILLIAMS
AND
BAFFLE
l
AIRDUCT I I
S N D~x~ I~l
A W VL
E
HE,G.T rm SCREENS
CONTROL 1 72~S--S------~-
............a FUEL
O-RINGSEAL
~ ~ j 'r'~ ~/jOVERFLOWDUCT
~ f~21/[..SJ~
WATER SPRAY~ quot;i quot;/////quot;//
SUCTION
SUCT'OI~I
EXHAUST l~J ~____~7~,
~,~L,I EXHAUST
RIG
CONTROL~j~ !!~ I1~~
FuWALTERAI~ ' / / ' /
N,
N ~ F L TE I
W A LINN
R
WATEROUT i
. ~ POOLDEPTH CONTROL
lOmm
SCALE , ,
FIG. 3. Schematicdiagram of diffusion-flameapparatus.
gently diffuses toward the fuel surface from the oxid-
values of the exit velocity U and of the ratio of oxygen
izing stream.
mass to total mass in the oxygen-nitrogen stream,
This behavior of the main constituents is roughly
Yo2, listed in the figure. There is a two-phase, gas-
consistent with the ideas of chemical equilibrium. The
liquid layer on the order of I mm thick at the surface
mixture ratio, measured on the basis of the ratio of
of the polymer under these burning conditions; the
location of the outer edge of this layer is indicated in carbon to oxygen or of hydrogen to oxygen, decreases
as the distance from the polymer surface increases. If
the figure, as is the location of the center of the
equilibrium calculations are made of temperatures
luminous blue zone, whose thickness is less than
1 mm. and of concentrations of 02, N2, C O 2 and H20 at
The monomer, methyl methacrylate, has the chemi- each point on the basis of the local mixture ratio, then
at least qualitative agreement with measurements is
cal formula CsH802 and is the major species liberated
obtained. There are quantitative discrepancies; for
in polymer pyrolysis (see Table 6). It is seen from Fig.
example the flame temperature is nearly 500K below
4 that this is the major fuel present at the outer edge
of the dispersed layer. This material diffuses into the the theoretical flame temperature. The magnitudes of
these discrepancies are indicative of the extent to
blue zone from below, while oxygen diffuses into the
blue zone from above. The heat release is greatest in which departures from equilibrium occur.
the center of the blue zone, where these two species As an extreme idealization, it may be considered
meet, as may be seen by the occurrence of the peak in that there is essentially no 02 on the fuel side of a
the temperature profile at the center of this zone. The sheet of negligible thickness located at zero, the center
concentrations of the major products CO 2 and H20 of the blue zone, and that there are essentially no fuel
species (CsHsO2, CO, HE, etc.) present on the oxygen
also peak near the center of the blue zone, and these
diffuse away on each side of this zone. Nitrogen, side of this sheet. This quot;flame-sheetquot; approximation is
which does not participate in the reaction, exhibits no useful conceptually as well as for approximate burning-
distinctive behavior at the blue zone but instead rate calculations, even though the information in Fig.

13. Urban and wildland fire phenomenology 329
POLY ( METHYL METHACRYLATE)
N 2 in 0 2
Yo2 =0.178
U = O.315m/s
300 9O
18
9 I,-.-
Z
w
~ 500 8O
8
16
I---
a.
Z
hi
,n,quot;
._1
o W
70 o_
~00
14 7 ~
W
quot;rquot;
J
O
m
re N
T
60 z
6 ~ ~.00
12
I--
&
z
Z
w
I
,,y (.)
rr
,i
w w
O_ 0_
50
)00
IO
w
t.d
J I
._1
O
0
~E
N T
O re
8 1- 4 ~ ;00
d
O
T
-rquot; re
U
O
3 r, O0
6 ~
-r
t~
U
T
2 a O0
4 u
T
U
O0
d
T
quot;r
I 0 I 2
DISTANCE FROM LUMINOUS FLAME ZONE (mm)
FIG. 4. Representative concentration and temperature profiles in a diffusion flame.
The pyrolysis of gaseous fuel proceeds in the dark
4 shows clearly that it is not very accurate in detail.
fuel-rich zone between the fuel surface and the blue
The flame-sheet approximation is a limiting form of
zone. Occurrence of the gaseous fuel species observed,
the equilibrium approximation.
rather than other fuel species, can be understood
The many species shown in lesser concentrations in
from concepts of kinetic mechanisms of pyrolysis of
Fig. 4, primarily on the fuel side, are not at all con-
C 5 H 8 0 2 .24 It is seen that many of the fuel species
sistent with chemical equilibrium. In addition to the
produced in dark-zone pyrolysis have higher ratios of
product CO of partial oxidation, these species include
carbon to hydrogen than the parent fuel.
the gaseous fuels hydrogen, methane, ethane, propane,
ethylene (C2H4) , acetylene (C2H2) , propylene (C3H6) ,
allene (CH 2 = C = CH2), propyne (CH3C = CH) and 5.5. Kinetics of Gaseous Fuel Pyrolysis
formaldehyde (HCHO). These latter species must be
Numerous chemical reactions occur in the dark
produced by finite-rate chemical processes. They are
pyrolysis zone containing gaseous fuel. These reactions
in no way representative of the species expected from
are complex and differ for different fuels; they are not
combustion kinetics, such as those discussed in Sec-
understood thoroughly. 25 If allowed to proceed for a
tion 5.1quot;, and they extend well beyond the blue
sufficient length of time, they result in production of
reaction zone. Instead, they are formed by pyrolysis of
soot. In the experiment of Fig. 4 there is insufficient
the secondary (gaseous) fuel C 5 H802. residence time in the fuel-rich zone for this to occur.
However, in Fig. 2 there is sufficient time, and the soot
becomes visible as the yellow zone of the flame. The
* More sophisticated experimental techniques are needed
soot also burns and finally is consumed completely at
to measure most of the nonequilibrium species of the com-
the upper boundary of the yellow region.
bustion kinetics.